Polyacetylene, its preparation and doping is described in U.S. Pat. Nos. 4,204,216 and 4,222,903. The disclosures of these patents are incorporated herein by reference.
Polyacetylene has valuable electrical properties for a wide variety of uses. These properties are enhanced by the doping of polyacetylene. However, when polyacetylene is prepared and doped, within a short time after its preparation and doping, the polyacetylene becomes brittle and also loses a portion of its enhanced electrical conductivity properties. Even when a polyacetylene powder is prepared and doped, the enhanced electrical conductivity of such doped powder decreases after a short period of time and the doped powder itself becomes modified so that the preparation of formed articles from the doped powder becomes difficult. One possible explanation for the loss of enhanced conductivity and the embrittlement of a doped polyacetylene formed material, such as a film is due to isomerization of cis-polyacetylene to trans-polyacetylene. However, it is known that cis-polyacetylene, although generally considered stable at temperatures of from about -78.degree. C. to 0.degree. C., does isomerize slowly, even at -78.degree. C., to trans-polyacetylene. At temperatures in excess of 0.degree. C., isomerization of cis-polyacetylene to transpolyacetylene is accelerated. During this conversion, free radicals may be formed which may crosslink or otherwise react with available oxygen. The reaction with available oxygen is believed to contribute to the embrittlement of, for example, a doped polyacetylene film by the formation of carbonyl and hydroxyl groups. These groups disrupt the conjugation of the polyacetylene double bonds and thereby decrease the enhanced electrical conductivity of the doped polyacetylene. Whenever cis-polyacetylene is isomerized to trans-polyacetylene, whether in a doped or undoped state, there will always be the formation of free radicals due to the isomerization mechanism. A discussion of the preparation of polyacetylene films and the isomerization of such films is set forth in the Journal of Polymer Science, Volume 12, pages 11 through 20, Shirakawa, et al (1974).
Embrittlement of a doped cis-polyacetylene film or formed article can be delayed by storing the doped film or formed article at a low temperature (-78.degree. C. to 0.degree. C.) under an inert gas such as nitrogen, argon or helium.
Although it is known that the cis-polyacetylene is more flexible than the trans-polyacetylene, the trans-polyacetylene has greater intrinsic electrical conductivity properties and the trans-form is thermodynamically more stable. The free radicals which may be formed during isomerization of cis- to trans-polyacetylene also trap oxygen and reduce the electrical conductivity of the doped polyacetylene (whether cis- or trans- if oxygen is present because it is believed that these free radicals form carbonyl and hydroxyl groups). Although, the state of the art is still such that the formation of these free radicals cannot be eliminated, if the presence of oxygen can be eliminated, then an aggravation of the results of free radical formation can be avoided. Thus, the problems of embrittlement and loss of electrical conductivity can be alleviated.
The previous practice of avoiding embrittlement involved preparation of cis-polyacetylene and storage of the cis-polyacetylene, whether doped of undoped, at low temperatures of from -78.degree. C. to 0.degree. C. under vacuum or an atmosphere of an inert gas. Such procedures are cumbersome in any practical ambient environment. Therefore, the utility of doped polyacetylene in applications requiring electrical conductivity is severely limited by the use of those procedures.
Any other approach to the aforesaid problem of the effects of oxygen must take into consideration the affinity of polyacetylene for oxygen. Thus, any material which would remove oxygen from the system must compete with the doped polyacetylene for the removal of such oxygen and must have a greater affinity for oxygen than the polyacetylene. Stated otherwise, any material which would remove oxygen must be able to compete successfully with polyacetylene for the oxygen present.
It is an object of this invention, therefore, to reduce doped polyacetylene crosslinking and embrittlement.
Another object of this invention is to provide a process for substantially preventing oxygen from contacting doped polyacetylene by providing a material which will successfully compete with the polyacetylene for the available oxygen.
Still another object of this invention is to provide a process for maintaining the electrical conductivity of polyacetylene.
A further object is to provide a doped polyacetylene composition having enhanced resistance to polyacetylene crosslinking and embrittlement.
Other objects and advantages will become apparent from the following more complete description and claims.